The use of a stylus with a touch screen interface is well established. Touch screen designs have incorporated many different technologies including resistive, capacitive, inductive, and radio frequency sensing arrays. Resistive touch screens, for example, are passive devices well suited for use with a passive stylus. The original PalmPilots® devices from the mid-1990s were one of the first successful commercial devices to utilize a resistive touch screen designed for use with a stylus and helped to popularize that technology. Although resistive touch screens can sense the input from nearly any object, multi-touch is generally not supported. An example of a multi-touch application may be applying two or more fingers to the touch screen. Another example may be inputting a signature, which may include simultaneous palm and stylus input signals. Due to these and other numerous disadvantages, capacitive touch screens are increasingly replacing resistive touch screens in the consumer marketplace.
Various capacitive stylus approaches have been implemented for use with touch screens and are found in many consumer applications such as point-of-sale terminals (e.g., the signature pad used for credit card transactions in retail stores) and other public uses. However, any type of capacitive stylus can be affected by the shadow effect which occurs to some degree at any non-perpendicular angle between the stylus and sensing area. The shape of the tip of the stylus as well as the materials used in its construction may exacerbate or mitigate this shadow effect.
FIG. 1 illustrates one embodiment 100 of a bullet-shaped stylus tip 103. Passive styluses usually have a tip shape that is similar to the point of a ball point pen. The entire tip of the stylus (the body of the tip) is made of a conductive material and it is that conductive material that affects the measured capacitance of the touchscreen and provides the location of the stylus. The user of a stylus, or a normal ball point pen, rarely holds the stylus vertically or perpendicular to the sensing surface. Rather, the pen and the tip are tilted toward the sensing surface, usually between 10 and 45 degrees. Different usage paradigms may lead to angles greater than or less than this range. As the stylus tilts toward the sensing surface, the reported position 144 moves away from the point of contact 142 between the stylus tip 103 and the sensing surface 110 in the direction of the tilt. The offset 143, the reported position 144 versus the point of contact 142, is attributed to the “shadow” of the stylus and may be greater than or equal to 1 millimeter. Such a disparity between the reported position 144 and the point of contact 142 may cause the stylus not to function as required by the program for which its use is intended. The “shadow” of the stylus is caused by the conductive tip of a bullet-shaped stylus having more capacitive coupling with the capacitance sensing electrodes in the direction of the tilt than with the capacitance sensing electrodes in the opposite direction. This phenomenon can be seen for both self-capacitance and mutual-capacitance touch screens. Another artifact of the bullet-shaped stylus tip is high self-capacitive coupling to the receive electrodes of a mutual-capacitance touch screens. Higher self capacitance coupling for the stylus tip to the receive electrodes of a mutual capacitance touchscreen may reduce the signal to noise ratio (SNR) according to Equation 1:
                    SNR        =                              Δ            ⁢                                                  ⁢                          C              m                                            C                          f              -              rx                                                          (        1        )            where ΔCm is the change in mutual capacitance that is caused by the presence of the stylus tip and Cf-rx is the self capacitance of the stylus tip to the receive electrodes of the mutual capacitance sensing array. By reducing the SNR of the stylus on the mutual capacitance sensing array, sensitivity of the sensing array to the stylus tip and by extension the usability of the stylus may be impacted.
FIG. 2 illustrates the field magnitude of a bullet-shaped stylus tip when it is in contact with a sensing surface. Longer, bolder arrays indicate greater field magnitude and greater capacitive coupling between the stylus tip and the sensing surface. Smaller, fainter arrows indicate lesser field magnitude and reduced capacitive coupling. An absence of arrows from the stylus to the sensing surface indicates an electric field small enough not to register during capacitance sensing. While there is some coupling in the direction opposite the tilt of the stylus tip, the magnitude of the electric field and the capacitive coupling between the stylus tip and the sensing surface is highest in the direction of the tilt. The increase in the magnitude of the electric field in the direction of the tilt versus the opposite direction causes the self capacitive coupling to the receive electrodes and the “shadow” effect, reducing SNR and causing the offset between the reported position and the actual position.